[llvm] [AMDGPU] Legalize SGPR hi16 copies into S_PACK_HH_B32_B16 (PR #218371)

[Jay Foad via llvm-commits]

jayfoad wrote:

In that pattern could you please try changing `(EXTRACT_SUBREG VGPR_32:$src1, hi16)` to `(EXTRACT_SUBREG (i32 (COPY_TO_REGCLASS $src1, VGPR_32)), hi16)`? That's what we do in other places.

https://github.com/llvm/llvm-project/pull/218371